1. Field of the Invention
The present invention relates to a system, an apparatus, and a method for loading bits into sub-channels. More particularly, the present invention relates to a system, an apparatus, and a method for loading bits into sub-channels according to the different sub-channel qualities.
2. Descriptions of the Related Art
Currently, most digital communication systems, such as orthogonal frequency division multiplexing (OFDM) and asymmetric digital subscriber line (ADSL), employ multi-channel or multi-carrier transmission. It is quite important in the field of multi-channel transmission to be effectively allocating bits into sub-channels so that the performance is able to be increased and the bit error rate (BER) during the transmission is able to be reduced.
Power and data rate are involved in a bit loading method, which allocates bits and power to sub-channels. Conventionally, the bit loading methods are either subject to a fixed power or a fixed data rate. When a bit loading method is subject to a fixed power, the method intends to maximize the data rate under the fixed power. When a bit loading method is subject to a fixed data rate, the method intends to minimize the power.
In U.S. Pat. No. 6,418,161, a bit loading method subject to a fixed power is provided, wherein the power is limited by a power spectral-density mask and an aggregate signal power constraint. The method allocates bits to sub-channels according to the lowest marginal power-cost per bit. If there is remaining aggregate signal power after all the bits have been allocated to the sub-channels with sufficient margins to carry a bit, additional bits are assigned to the sub-channels using frequency-domain-spreading across several sub-channels at the corresponding reduced power levels. Although optimal, this method is too complicated for initial bit allocation.
On the contrary, the bit-filling method and the bit-removal method are subject to a fixed data rate. Both of them allocate or remove bits iteratively based on the power cost of sub-channels. The bit-filling method initially allocates zero bits to all sub-channels and then adds one bit at a time to the sub-channel that requires minimum additional power until the target data rate is achieved. The bit-removal method initially allocates maximal bits to the sub-channels and then removes one bit at a time from the sub-channel that reduces the maximum power until the target data rate is achieved. Both the bit-filling and bit-removal methods result in the same bit allocation. Although both methods can achieve optimal results, they are complicated and inefficient due to the bit-based iteration.
Although optimal bit loading methods have been proposed in this field, all of them are extremely complicated. Thus, an efficient approach for allocating bits to sub-channels is still needed in this field.